The present invention relates to a high-frequency, low-loss elastic surface wave device featuring a superior band-pass characteristic shape factor, stop-band rejection capability and ripple characteristic.
Elastic surface wave devices having a transducer configuration in which apodized transducers and unapodized transducers are combined to obtain desired amplitude-frequency characteristics have been most commonly used because characteristics of the total system can be given easily as a product of the amplitude-frequency characteristics of each transducer incorporated in the device. Hence, it is easy to design elastic surface wave devices having such a transducer configuration. Aside from such a conventional transducer configuration in common use, a phase-weighted transducer configuration comprising a group of transducers all arranged with an equal overlapping length on the same propagation path has also been proposed; and a transducer configuration in which such phase-weighted transducers are substituted for apodized transducers or in which such phase-weighted transducers, instead of unapodized transducers, are combined with apodized transducers has also been in use in some sectors.
Of the foregoing conventional types of elastic surface wave devices, those employing a transducer configuration in which unapodized transducers and apodized transducers are combined have the following disadvantages:
(1) It is difficult to adequately suppress stop-band response signals. Particularly, if a small number of pairs of high-impedance unapodized transducers with a bandwidth sufficiently larger than required are used for an elastic surface wave device, the desired stop-band rejection has to be achieved only with apodized transducers. As a result, a larger number of pairs of apodized transducers are required and hence a larger substrate area is required. Furthermore, such disadvantageous features of the elastic surface wave device result in greater effects of diffraction caused where transducer overlapping lengths are small, to eventually allow stop-band response signals to grow larger. If unapodized transducers with a bandwidth which is not so much larger than required by the elastic surface wave device are used, the effects of their traps (poles) and side lobes promote stop-band rejection for the entire device; but since the effect of diffraction cannot be totally eliminated, stop-band rejection cannot be promoted as much as expected depending on the case. The problem of stop-band rejection assumes still greater significance particularly for a low-loss elastic surface wave device to be used in a state in which its transducers are nearly matched with the power supply or load. At lower-order side lobes near the pass band, the radiation conductance of transducers is not necessarily small enough with respect to the internal conductance of the generator, and also the imaginary part (susceptance) is canceled in a state in which conjugate matching is nearly achieved; so that the relative magnitudes of lower-order side lobes noticeably increase.
(2) The impedances of unapodized transducers cannot be set as desired. Particularly, if a high frequency filter configuration with a small fractional bandwidth is employed to attempt stop-band rejection enhancement using unapodized transducers with a bandwidth which is not so much larger than required, the number of transducer pairs required increases. Furthermore, in such a case, to avoid excessive growing of the effects of diffraction waves generated where apodized transducer overlapping lengths are small, the aperture cannot be narrowed; so that greater radiation conductance and susceptance result. If, for example, a band-pass filter system with a center frequency f.sub.0 of 402.78 MHz and a 3-dB attenuation bandwidth of 30 MHz is created using a lithium niobate (LiNbo.sub.3) single crystal substrate of 128.degree. rotated Y-axis cut and X-axis propagation; 59.5 pairs of apodized transducers and 13 pairs of unapodized transducers are required, and it is necessary to use split fingers. If the aperture is set to 300 .mu.m in this case, the radiation conductance of apodized transducers will be about 4 mS and that of unapodized transducers about 6 mS. If such a filter is used in an ordinary two-transducer configuration with an outer-circuit conductance of 20 mS (for a 50-.OMEGA. system), the so-called regeneration by the load or power supply will cause multiple transit signals to be actively generated, to eventually allow heavy rippling. The stop-band rejection magnitude in this case will be only about 32-34 dB at a frequency about 40 MHz off the center frequency; the stop-band rejection deterioration caused by diffraction is about 10-15 dB with respect to the design value. Under such conditions, it is impractical to further narrow the aperture, so that it is impossible for practical purposes to reduce the radiation conductance. If a three-transducer configuration is adopted for loss reduction, the central transducer has to be matched with the outer circuit, whereas, for the outer transducers, it is necessary to reduce their radiation conductance to about one-tenth of the outer-circuit conductance in order to suppress multiple transit signals in the entire bandwidth. If the outer-circuit conductance is 20 mS, standard value, in such a case, the central transducer can be matched with the outer circuit by using an impedance matching circuit network or by appropriately adjusting the aperture length; but the radiation conductance of the outer transducers cannot be set to a desired value without using a circuit network which causes impedance mismatching. Such condition is to be observed whether apodized transducers and unapodized transducers are used as the central transducers or as the outer transducers. Using a small number of pairs of unapodized transducers as the outer transducers enables the radiation conductance of the outer transducers to be set as desired, but it makes stop-band rejection more difficult.
What are regarded as phase-weighted transducers making up the conventional elastic surface wave devices can be classified into the following three categories:
(a) Those consisting of a number of unapodized transducer groups which are equally spaced and are connected in series or parallel, with each group including the same number of pairs of unapodized transducers. This category of phase-weighted transducers include those described in the following information;
(a-1) U.S. Pat. No. 3,550,045 (R. Adler) PA0 (a-2) U.S. Pat. No. 3,600,710 (R. Adler) PA0 (a-3) U.S. Pat. No. 3,825,860 (Paul H. Carr) PA0 (a-4) 1972, IEEE Ultrasonics Symposium Proceedings, p. 218-220, Alan J. Budreau and Paul H. Carr, Narrow Band Surface Wave Filters at 1 GHz. PA0 (a-5) U.S. Pat. No. 3,846,723 (Philip L. Writer, et al.) PA0 (b-1) U.S. Pat. No. 3,792,381 (T. W. Bristol) PA0 (b-2) 1972, IEEE Ultrasonics Symposium Proceedings, p. 377-380, T. W. Bristol, Synthesis of Periodic Unapodized Surface Wave Transducers. PA0 (c-1) Japanese patent application laid open No. 14093 of 1974 (Sekine) PA0 (c-2) U.S. Pat. No. 3,946,342 (C. W. Hartmann) (Japanese patent application laid open No. 40259 of 1975) PA0 (c-3) 1973, IEEE Ultrasonics Symposium Proceedings, p. 423-426, C. S. Hartmann, Weighting Interdigital Surface Wave Transducers by Selective Withdrawal of Electrodes.
(b) Those adopting a transducer configuration which comprises unapodized central transducers, on both sides of which other unapodized transducers are symmetrically arranged with equal spacing from the central position of the central transducer group. This category of phase-weighted transducers include those described in the following information;
(c) Those comprising transducers which overlap each other with an equal overlapping length, with parts of fingers withdrawn. This category of phase-weighted transducers include those described in the following information;
Of the foregoing three categories of existing phase-weighted transducers, those belonging to the category (a) all concern simple, narrow-band filter configurations or filter configurations with a bandwidth including peaks and troughs. They do not incorporate any consideration for stop-band response signal suppression, shape factor improvement for the pass-band frequency response, frequency response improvement by means of a combination of phase-weighted transducers and apodized transducers, and loss reduction; so that such improvements cannot be realized by them.
Next, those belonging to the above category (b) adopt a transducer configuration comprising central unapodized transducers adjoined by reverse-phased unapodized transducers on both sides. Such a transducer configuration is aimed at canceling the primary side lobe of unapodized transducers while enhancing the flatness of the band-pass characteristic. Generally, however, enhancement of stop-band rejection on transducers, by means of canceling the primary side lobes generated by unapodized transducers, and improvement of the flatness of the band-pass characteristic cannot be pursued at the same time; hence a problem with the above-mentioned transducer configuration is that it enables both targets to be pursued at a time only when a particular pair ratio is realized between the central unapodized transducers and other unapodized transducers adjoining the central transducers on their both sides. Furthermore, since the bandwidth is determined by the number of pairs of central unapodized transducers, the radiation conductance cannot be easily controlled. In other words, a smaller fractional bandwidth for high frequencies results in larger radiation conductance as previously mentioned. Therefore, the transducer configuration identified with the foregoing category (b) is not suitable when a small high-frequency, fractional bandwidth is involved.
Lastly, it is very difficult to design the phase-weighted transducers belonging to the foregoing category (c), because it is not known how to control the radiation conductance and bandwidth, or how to determine the band shape for them. They have another drawback which is common to all the three categories of transducers. That is, the difference in elastic surface wave speed between the locations where unapodized transducers are arranged at equal pitches and where fingers are removed is not taken into account; so that there are cases in which their applications result in bandwidth characteristic shape and frequency errors.